The present invention relates generally to material processing using photon energy and more specifically to a method and system for high photon energy material processing using a liquid core waveguide.
During material processing, such as machining, thermal treatment, and laser shock peening, for example, high intensity energy sources, such as photon energy sources, are often implemented. While using a photon energy source, such as a laser, the photon energy may be transmitted through a medium called a waveguide and directed towards a target or material to be processed. In order to make the process efficient and avoid over-heating the waveguide, the energy loss during the transmission of photon energy should be very low. A principle, called total internal reflection (TIR), facilitates efficient transmission of photon energy through waveguides. Total internal reflection (TIR) is a phenomenon by which an electromagnetic wave is completely reflected when it travels from a medium of high refractive index to a medium of lower refractive index with an incident angle greater than a critical angle.
In order to meet the criteria for TIR, a solid core waveguide consisting of a solid core fiber having a high refractive index surrounded by a cladding having a low refractive index is often used in photon energy transmission. The use of a solid core fiber is effective in the telecommunication industry, which uses photon energy on the order of less than 1 watt/square centimeter (w/cm2), which is several orders lower than that used in material processing, which is typically higher than 104 w/cm2.
Disadvantageously, solid core fibers have inherent difficulties in transmitting high peak energy intensities or high average powers due to the presence of defects in solids. The defects in the solids scatter and absorb the incident photon energy, and thus, the defects act as local heat centers. When either the peak energy intensity or the average power is high, the solid core fiber distorts so much that the TIR condition is destroyed, and the solid core fiber eventually breaks down. Lasers with nanosecond or even shorter pulse durations are widely used in research and industry, but currently the solid core fibers cannot be used for such pulsed lasers due to the energy limitations mentioned above. Improving the purity of the solid core fiber may improve the transmission of photon energy using a solid core fiber. However, even with improved purity, solid core fibers pose some limitations. For example, even with a high purity ruby, the intensity of laser that can be transmitted through a solid core fiber is typically lower than 108 w/cm2. Furthermore, improving the purity of the solids also increases the cost of the solid core fibers.
Another method of transmitting photon energy is to use a hollow waveguide. A hollow waveguide relies on reflection of photon energy by smooth surfaces. Unlike TIR, each air-solid reflection has certain energy loss. Disadvantageously this makes the method of using a hollow waveguide less efficient and thus, introduces certain application limitations. Further, hollow waveguides also have the same limitations as solid core fibers, since any solid material may be damaged when exposed to high energies. Even glass, which is generally transparent to laser energy, may be damaged when exposed to energies greater than 109 w/cm2.
Furthermore, material processing using high-intensity photon energy is accompanied by thermal effects, which may be undesirable. For example, in laser machining using nanosecond or longer pulse durations, the machined region has a heat-affected zone (HAZ), which usually has tensile stress distributions. Thus, in addition to the problems set forth above, laser machining in air or a vacuum may also result in melting of the target, re-deposition of the target residue and attachment of the residue that may require post-processing.
Yet another method of transmitting photon energy is to use a liquid medium, such as a water jet in air, to transmit and confine an energy beam. Disadvantageously, the length of energy transmission is limited and the TIR effects due to water-air interface disappear once the water jet hits the target. Furthermore, bubbles will be formed during laser material processing. As will be appreciated, bubble formation generally lowers the process quality and limits its application. The bubble issue and the length issue mentioned above are not well solved in water jet laser energy transmission.
Thus, there exists a need for an improved method and system for material processing using photon energy techniques. More specifically, there is a need for an improved waveguide for flexibly transmitting high intensity photon energy and a method for improving the quality of material processing.